Traditionally, as a countermeasure against exhaust gas of diesel engines and the like, there has been a technology that adopts an EGR device (exhaust-gas recirculation device), which circulates a portion of exhaust gas to an intake side, to keep the combustion temperature low, thereby reducing an amount of NOx (nitrogen oxide) in the exhaust gas.
Examples of such a type of EGR device are disclosed in Patent Literature 1 to Patent Literature 4 (hereinafter referred to as PTL 1 to PTL 4, respectively) and the like. In each of the EGR devices as described in PTL 1 to PTL 3, a recirculation flow pipe passage branched off from an exhaust manifold of a diesel engine is connected to an intake manifold. By supplying a portion of the exhaust gas (EGR gas) to the intake manifold through the recirculation flow pipe passage, the EGR gas is mixed with fresh air from the intake side, the mixed gas is introduced into cylinders (cylinders in an air intake step) of the diesel engine.